Woven fabric diaphragm with stiffened portions



April 4, 1950 J. E. KEDDIE 2,502,853

, WOVEN FABRIC DIAPHRAGM WITH STIFFENED PORTIONS Filed Feb. 3. 1948 1' l I Q 2 I v o n 2 2 E/ I i B }J James Edwavcl. Kecldie AGENT.

Patented Apr. 4, 1950 UNITED STATES ATENT OFFICE 'WOVEN FABRIC DIAPHRAGM WITH STIFFENED PORTIONS Application February 3, 1948, Serial No. 5,991 In Great Britain November 8, 1945 (C1. ISL-32) 2 *Claims.

This invention relates to electro-acous'tic apparatus employing a diaphragm in the form of acone, such as cone loud-speakers, and "has more particular reference to the cone diaphragm and supporting surround in such apparatus.

According to the present invention, the cone proper and the supporting surround are made from a continuous piece of woven fabric the threads of which are each constituted by anex- 'truded or drawn thread or by a plurality of extruded or'drawn filaments, the cone iproper being treated'so as to interlock the threads'of the "fabric to produce a substantially rigid'structure whereas the surround portion is left untreated so that the crossing "threads in the surround slide freely relatively to each other at each intersection point apart from the restraint imposed by adjacent threads. The threads or filaments are'preferably of glass, but use may be made of other materials, such as synthetic plastics, yielding drawn or extruded filaments or threads exhibiting a smooth surface. The interlocking of the threads is conveniently achieved by the application of a'binder or dope.

"The untreated fabric surround in the rest position of the cone preferably takes the form of a toroidal surface which may be convex or concave as viewed looking on the open end of the cone, the'surface of the cone being'tangential to the toroidal surface.

.At or adjacent the apex, the conical diaphragm may be supported in well-known manner by a spider engaging at the coil neck for example. Preferably the spider is constructed in a manner analogous to the cone.

In order that the invention may be more readily understood, reference will now be made to the accompanying drawing, which shows in central section. a preferred construction of loud-speaker according to the invention.

The loud-speaker comprises a rigid -framework or chassis i .to which the magnet assembly is secured-by means of bolts 1 and a clamping member 8. The magnet assembly comprises a ring .2 surrounding a pole-piece 3, the annular gap between the ring 2 and the pole-piece 3 providing the magnetic field in which the speech call I 0 moves. The pole-piece 3 is mounted on a permanent magnet 4 and is centered relatively to a pot 5 by means of a centering ring 5. The coil I0 is wound on a coil former 9 by means of which the coil is secured to the loud-speaker cone H which has a cylindrical collar or neck 52 embracing the end of the coil former 9. The cone II, the cone neck I2 and the cone surround I3,

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ployed in the manufacture of fabric cone loudspeakers, such as a cellulose acetate binder. Between the outer edge of the cone ll and the inner edge of the marginal rim M, the glass fibre cloth is untreated and is given the shape of a part toroidal surface which is convex towards the open end of the cone, the cone being tangential to this curved surface at its inner extremity and the'rim M being tangential to it at its outer extremity. The outer edge of the rim M is secured by embedding in a fixative in an annular trough l5 formed adjacent the outer periphery of the chassis l. The fixative preferably has a solvent different from that of the binder used for doping the glass fabric as a similar solvent might soften the outer rim during the setting period. The un" treated part it is a substantially part toroidal in the rest position, and on displacement of the cone l 5 under the action of the currents in the speech coil H) in the magnetic field, the shape of the part it varies but as the threads are free to slide relatively to one another there is no elastic deformation of the surround and energy dissipation in the surround is primarily frictional, provided that the range of movement of the cone lies within the capacity of the surround. The limits thus imposed may readily be arranged to lie outside the largest movement required to cover the acoustic reproduction range for which the speaker is designed. As an alternative to the arrangement as shown, a flat type edge may be used in which event a gasket may be inserted between the chassis member and the rim of the cone, and the whole assembly of chassis member, gasket and rirn glued together.

.3 rigid central portion l6 carrying a rigid collar 11 embracing the coil former 9. The rigid central portion 16 is a substantially fiat disc and may be in the form of a spoked wheel as in conventional spider arrangements. According to the invention, the rigid central portion i6 is surrounded by a rolling undoped portion I8 serving the same function as the toroidal part I3 of the cone surround. The undoped portion 18 is followed by a marginal portion 19 which has been doped to render it rigid and is secured by fixative in an annular channel 20 adjacent the inner periphery of the chassis l.

In constructing a loud-speaker as shown in the drawing, the cone and the spider are first separately shaped and the appropriate parts are doped. The pro-formed cone and the pre-formed spider are then associated with each other and with the speech coil to form an assembly which is fitted into the chassis, the outer edges of both cone and spider being cemented into the chantributed to the fact that normally almost all.

sounds to be reproduced start at the lowest audi- -ble frequency so far as the ear is concerned and pass up the scale to the sustained note. The transition phase may be very rapid and postulates the requirement that the cone assembly must follow accurately the frequency changes that occur. If the frequency change passes through a natural resonance frequency of the loudspeaker there will be a delay in the rising frequency response. quick rise to the sustained note. The aural effect which results is a slurring of the commencement of the sound which is being reproduced. The clarity of reproduction is improved not only due to this absence of or diminution in the slurring effect, but also due to another property of the laud-speaker. Considering the simple case of a high frequency note superimposed on a low frequency note if the loud-speaker has inertia at resonance frequency and if during reproduction of the low note the cone assembly has large movement and high velocity, then the small power change of the high note becomes smoothed out in the movement of the cone and is not reproduced in proportion to the input value. In the case of the resistance surround cone assembly according to the present invention, there is no stored inertia since the energy is dissipated in friction in the surround, so that the cone is always under electrical drive from the speech coil and any changes in the drive are reproduced in .the cone assembly. Consequently, the cone assembly according to the invention contributes to the input being followed to a more faithful degree than with cone loud-speakers of the freely suspended type.

It will be noted that in the construction de scribed with reference to the drawing, neither This is followed by an overi the cone nor the spider produces a centering force as in conventional arrangements. The glass fibre cone possesses more mechanical freedom than a conventional cone, and therefore for the same input current the initial amplitude and acceleration are greater in the glass fibre cone. Due to the greater mechanical freedom, harmonic damping and time lag are reduced so that reproduction faults are diminished. It is found in operation that the centre of the speed coil tends to line up about the centre of the magnetic field in the gap in which the speech coil. moves. This can be attributed to the fact that the field in the gap is not uniform but is greatest at the centre of the gap, and as the movement of the speech coil is centered round the strongest field, the speech coil tends to vibrate about a mean centre of flux density.

I claim:

1. An acoustic diaphragm device comprising a. diaphragm having a cone-shaped surface and a supporting surround integral therewith, said diaphragm and surround being of a continuous piece of a woven fabric, the portion of said woven fabric which forms said diaphragm being fixed by a binder while the portion of said woven fabric which forms said surround is untreated in a first annular region contiguous to said diaphragm but fixed by a binder in a second annular region remote from said diaphragm, said first annular region having in the normal rest position a toroidal surface which is convex in the direction towards the open end of said cone-shaped diaphragm, said cone-shaped surface of said dia- F phragm being tangential to said toroidal surface of said untreated annular region, a coil former attached to said diaphragm in the region of the apex thereof, and a supporting spider attached to said coil former, said spider comprising a second integral piece of said woven fabric which has a first annular region contiguous to said coil former fixed by a binder, a second annular region contiguous to said first annular region of said spider which is untreated, and a third annular region contiguous to said second region and remote from said first region and fixed by a binder, said second annular region having in the normal rest position a toroidal surface which is convex in the direction towards the open end of said cone-shaped diaphragm, the portion of said first annular region immediately contiguous to said toroidal surface of said second annular region being tangential thereto.

2. An acoustic diaphragm device as claimed in claim 1 wherein said woven. fabric comprises glass threads.

JAMES EDWARD KEDDIE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

